1. Field of the Invention
The present invention relates to the correction of carrier frequency offsets in a mobile communication network.
2. Description of the Related Art
FIG. 1 illustrates a prior art receiver arrangement for receiving a CDMA signal. An analogue oscillating signal on a radio frequency is received through an antenna 2, downconverted onto a complex base band frequency signal in a receiver 4 and converted into a series of digital samples in an A/D converter 6. In the present CDMA receiver, despreading of the signal is performed by first supplying samples to a multiplier 10 for multiplying the received samples by the complex conjugate of a long code (referred to as “a scrambling code” in WCDMA). The scrambling code is used to separate different users. The signal from the multiplier 10 is supplied to a further multiplier 16 for multiplying the signal with a short code. The short code is used to separate the control channel from the data channel. The resulting despread data signal is then integrated in an integrator block 18.
The signal from the multiplier 10 is also supplied to a further integrator 12, the integrated control symbols being supplied to a channel estimation block 14. The channel estimation block 14 estimates the complex channel coefficient of the radio channel using pilot signal information (or a training sequence in TDMA receiver), and provides a channel estimate for removing the effects of the radio channel from the received signal. The signal from the integrator 18 is multiplied by the complex conjugate of the channel estimate at multiplier 20. The resulting output includes recovered data in the form of soft symbol data with reliability information associated with the recovered data.
The output is further transformed into a real signal in block 22, and then supplied to subsequent de-coding operations. It will be appreciated that the imaginary part may also include data, in which case the invention could likewise be applied to the imaginary part. A problem that exists with the system illustrated in FIG. 1 is the existence of frequency errors, or “carrier frequency offsets” in the received signal. There are two main causes for such frequency offsets. The first one is a frequency offset in the receiver oscillator, which is used for downconverting the received RF signal. This means that there is a frequency offset between the receiver oscillator frequency and the carrier frequency of the base station. This offset, for example, degrades the performance of the channel estimator.
Another cause for rotation of the signal is the so-called Doppler effect. This means that the length of the radio signal path between the mobile station and the base station changes when the mobile station moves. This causes a Doppler shift in the spectrum in the received signal. A mobile station synchronises its system clock according to the received signal from the base station. For a moving mobile, the Doppler effect will modify the observed carrier frequency. Therefore the mobile will end up transmitting the wrong carrier frequency. When the base station receives the signal the Doppler effect has again modified the carrier frequency in the same direction so the base station observes a carrier frequency offset that is two times the Doppler shift.
In mobile station receivers, the frequency offset may be detected and the frequency of the local oscillator can be controlled to remove the offset. However, this relies on information being available as to the exact amount of the frequency offset. Another problem is that the resolution of the oscillator frequency adjustment is usually too coarse to adequately compensate the frequency offset. In base station receivers it is not possible to adjust the local oscillator frequency, because the local oscillators are common for several channels and the frequency offset is usually different for signals that were received from different mobile stations.
A significant problem with the carrier frequency offset is that it shifts the power spectrum of the received signal. Because of the Doppler shift, the spectrum is no longer symmetric so, a channel estimator with real valued filter coefficients no longer works in an optimum way.
One existing approach to deal with this problem is that the carrier frequency offset is not corrected at all. The main idea here is to choose filter coefficients with a large enough passband so that the actual signal is not filtered away in channel estimation. This has very poor performance with large carrier frequency errors.
In the system illustrated in FIG. 1, the effective carrier frequency offsets can be corrected by the use of complex valued filter coefficients in the channel estimator 14. This allows for the compensation of the static rotation of the channel in the filtering step used to estimate the channel. The required computation depends on the chosen filter, but is in any event quite large. It is often not feasible to implement the required computations in a practical system with adequate performance.
Another possibility for correcting the carrier frequency offset is to monitor the phase of the channel estimate and generate a complex phasor on the basis of successive phase values. The resulting phasor is then used to correct the received base band signal before or after channel estimation. The carrier frequency offset is removed by de-rotating samples both after descrambling with the long code and descrambling with the short code. A disadvantage with this method is that the computations which are required depend on the data rate. Also, the number of computations which is required is so high that it is not feasible to implement this with DSP software in a practical system. Such a system is discussed in EP-A-1160981.